This invention relates to a signal processing circuit for use in a control system of an internal combustion engine, the processing circuit in use receiving a first series of signals from a first transducer, the signals of the first series occurring at relatively small intervals of engine crankshaft rotation and a second series of signals from a second transducer, the signals of the second series occurring at relatively large intervals and serving as engine cylinder identification signals, the signals of the second series including a further signal which occurs a predetermined time before one of the engine cylinder identification signals and which acts as an engine position signal.
The supply of the first and second series of signals to an internal combustion engine control system is well known in the art and the control system on the basis of the above signals and further signals supplied to it determines in the case of a compression ignition engine, which cylinder is to receive fuel and the timing and duration of fuel delivery.
In a known system the first transducer is associated with a disk or wheel which is driven by the engine crankshaft and the periphery of the disk defines teeth which are spaced at 6 degree intervals. The second transducer is associated with a disk or wheel which in the case of a four stroke engine is driven at half engine speed by the camshaft of the engine, and has a number of equi-spaced teeth equal in number to the number of engine cylinders with an additional tooth to provide the engine position signal, the additional tooth being say 15.degree. in front of one of the aforesaid teeth.
Modern control systems are microprocessor based and it has been the practice to separate the cylinder identification signals and the engine position signal using suitable software. In order to ease the software overhead it is proposed to separate the so-called "N" signals which identify the cylinders and the so-called "+1" signal which represents the engine position, in a hardware signal processing circuit.